As an improvement over the spray painting of articles such as automobile bodies and household appliances, a new kind of sheet material has been developed to provide protective and decorative coatings. The new material comprises a thin, flexible, stretchable, thermoplastic support sheet, known as a carrier film, which has a protective and decorative paint layer of uniform thickness on one side and, optionally, an adhesive layer on the other side. It can also have other layers such as a tie or bonding layer between the paint and the carrier film and a clear coat over the paint or basecoat layer. Using the known procedure of thermoforming, the sheet material can be stretched and bonded to an article such as an automobile body panel. Important advantages over spray painting include economy in the use of paint and avoidance of air pollution by evaporating solvents. Furthermore, the new material has a remarkably more attractive appearance than spray painted finishes.
The new type of sheet material and a process for its manufacture are described in the G. G. Reafler U.S. patent application Ser. No. 116,426, filed Nov. 3, 1987. The process involves providing a laminar flow of the coating composition on the surface of the thermoplastic carrier film to form a layer of substantially uniform thickness, followed by a drying procedure, then coating and drying each additional layer in sequence to obtain a finished product of excellent gloss and smoothness.
In manufacturing the new type of sheet material each coating is dried before the next is applied. Thus, the tie coat is coated and then dried before the paint layer is applied and the later is dried before the topcoat is coated and then dried. To obtain films with superior surface qualities each drying operation preferably is carried out slowly.
As an improvement in the manufacture of the sheet material, a method of drying the paint layer has been developed which reduces the occurence of bubble defects. It has been described in the copending Fronheiser et al U.S. patent application Ser. No. 166,083 filed Mar. 9, 1988, now U.S. Pat. No. 4,872,270. Although this drying method is highly effective, it does require lengthy drying periods, e.g., of the order of 10 minutes or more for the final stage drying and curing of the paint layer.
A principal reason for the slowness of the drying operation is that the carrier film, which necessarily is thermoplastic so that it can be stretched during thermoforming to automobile parts, must be dried at a temperature below its thermal deformation temperature. At such moderate temperatures the drying time is lengthy and adds to the cost of the process.
Furthermore, it has been found that in drying water-based paint layers which also contain high boiling organic solvents, moderate temperatures do not reduce the residual solvents to the lowest possible levels even when the drying time is lengthened. Retention of residual high boiling solvents can adversely affect the adhesion of the layers to the carrier film and possibly also the surface qualities of the coated film. A need has existed for a method which shortens the drying times for each coating or permits higher final drying temperatures, or both, and which reduces the residual solvent content to desirably low levels. In accordance with the present invention such a method is provided.
The extrusion of a polymer film onto another film is known. Furthermore, others have proposed to form a releasable lamination of a very flexible and easily deformable film and a stronger film in order to apply paint coatings to the easily deformable film by knife coating or the like. Such procedures, however, do not suggest that the drying of a layer coated on a heat deformable carrier film can be improved by releasably laminating the carrier film to a heat resistant temporary carrier film, then applying the layer by laminar flow coating and drying it initially at a moderate temperature and finally at a high temperature above the heat deformation temperature of the carrier film.
The Backwell U.S. Pat. No. 3,620,872 discloses a method for making a multilayer web material wherein a polymer coating is applied to a stable carrier web such as a polyester film. The patent offers no suggestion of providing a way to increase the drying stage temperature and shortening the drying time for a paint-coated stretchable, thermoplastic film.
The Jones et al U.S. Pat. No. 4,233,358 discloses a method for the production of stretchable coated textile fabrics and involves a transfer coating technique. Again there is nothing to suggest a way of reducing the drying time and the residual solvents content for a paint-coated thermoplastic film.
The Tieniber U.S. Pat. No. 3,650,880 discloses the manufacture of a porous polymer-coated fabric by applying a polyurethane film to a carrier member, applying a second film to the first one and then a fabric material. The carrier member is stripped from the first film layer. The carrier is a metal band or a belt of fabric or paper (col. 1, line 46) and is said to be "heat resistant so as not to be affected by the drying heat." (col. 1, lines 50-51.) There is no suggestion of making a sheet material of the kind made by applicants nor of the drying of a paint layer initially at a moderate temperature and then finally at a high temperature to a very low residual solvent content.